Spark plug and method for producing it

ABSTRACT

A spark plug for an internal combustion engine, the spark plug having at least two electrodes, in which the electrodes are formed of a first part made of a substrate material and a surface part more durable than the substrate material. The surface part of the spark plug is joined to the first part via an intermediate part making use of explosion welding. The method also relates to a method for producing a spark plug for an internal combustion engine, the spark plug having at least two electrodes.

This is a national stage application filed under 35 USC 371 based onInternational Application No. PCT/FI2004/050025 Filed Mar. 10, 2004, andclaims priority under 35 USC 119 of Finnish Patent Application No.20030401 filed Mar. 18, 2003.

The present invention relates to a spark plug comprising at least twoelectrodes, the electrodes being formed by first part made of asubstrate material, and a surface part, more durable than the substratematerial. The present invention also relates to a method for producing aspark plug for an internal combustion engine, the spark plug having atleast two electrodes.

Spark plugs are generally used in e.g. internal combustion engines forigniting the mixture of fuel and air/oxygen in the combustion chamber.This is accomplished by introducing a relatively high voltage differencebetween the electrodes of the spark plug, whereby a spark igniting themixture is formed.

Currently, the development of the spark plugs of internal combustionengines is more and more based on utilizing metals and metal alloys ofthe Pt group (Pt, Pd, Ir, Rh, Ru, Os) having a high melting point andgood resistance to oxidation erosion and spark erosion. This developmenthas been more pronounced especially with the increased service life andreliability requirements of the gas engines having pre-chambers. Forexample, the publications U.S. Pat. No. 6,078,129 and JP 11-003765disclose the use of an iridium alloy belonging to the Pt group.

It has been estimated that during ignition the surface temperature ofthe electrodes is in the range of 2000-3000° C., so a good resistanceto, for example, oxidation is required of the material. The largechanges of temperature also require a high resistance against, forexample, the thermal fatigue of the joints. In publication U.S. Pat. No.6,078,129, a part made of iridium alloy has been fastened to the centreelectrode of the spark plug by means of laser welding. Melt welding,however, produces a joint surface that is not optimal for thisapplication. A melting zone is produced in the method, requiring alarger material thickness of the joined part. Further, brittle areas canbe formed in the interface surface and it can become relativelyinhomogeneous.

The electrode materials must also have a good resistance to corrosion.Resistance to corrosion and oxidation together enhance improvement ofwear resistance, as wear usually occurs through spark erosion and/oroxidation wear. It is known that the service life of metals having ahigh melting point and belonging to the Pt group, such as iridium andplatinum and their alloys, in such applications is usually sufficientlylong. Especially iridium and its alloys with, for example, rhodium orrhenium, has considerably enhanced the increase of the service life ofspark plugs. Rh and Re mainly increase the oxidation resistance ofiridium. The construction and method of production of a spark plug isnot, however, currently very preferable.

The present invention relates to a spark plug for an internal combustionengine minimizing the problems of prior art. The invention especiallyrelates to a spark plug for an internal combustion engine achieving along service life. The invention also relates to a method of producing aspark plug for an internal combustion engine, by means of which thedesign of the spark plug is durable and the production process isinexpensive.

In a spark plug for an internal combustion engine having at least twoelectrodes the electrodes are formed of a first part, made of asubstrate material, and a surface part, more durable than the substratematerial. A characterizing feature of such a spark plug is that thesurface part is joined to the first part by means of an intermediatepart and that the joint between the surface part and the intermediatepart is an explosion weld joint. Thus, the depletion of the surface partinto the substrate material can be minimized. Correspondingly, the jointbetween the substrate material and the intermediate part is preferably aconventional melt metal joint. This in turn allows the joining of theelectrode to the surrounding construction, such as the body of the sparkplug, to be carried out more easily and reliably than in prior art. Thesubstrate material acting as the first part can be a part of the sparkplug body, especially in the case of a ground electrode.

In an electrode according to the invention, the surface part ispreferably formed of one or more of the metals of the Pt group (Pt, Pd,Ir, Rh, Ru, Os) or an alloy thereof. The joint between the surface partand the intermediate part is preferably homogenous on the area of thewhole joint surface, whereby it is also strong.

Preferably, in an electrode according to the invention the thickness ofmaterial of the surface part perpendicular to the surface of the jointbetween the surface of the electrode and the intermediate part is 0.05-2mm.

According to the invention, the method for producing a spark plug havingat least two electrodes, in which spark plug the electrodes are producedfrom a first part, made of the substrate material of the spark plug, anda second part, made of a material more durable than the first material,comprises the following combination of production stages, in which

-   -   a blank is formed, comprising a surface part and an intermediate        part, by joining the surface part to the intermediate part by        means of explosion welding,    -   a part with a suitable form is separated from the blank to form        the electrode of the spark plug, and    -   the part separated from the blank is fastened to the first part        of the spark plug so that the joint is made between the said        first part and the intermediate part.

According to one embodiment the surface portion of the blank ispreferably formed of a planar piece consisting of one or more metals ofthe Pt group or an alloy thereof, the piece being explosion welded tothe equally planar intermediate part.

According to another embodiment the surface portion of the blank ispreferably formed of a powder consisting of one or more metals of the Ptgroup or an alloy thereof, the powder being simultaneously solidifiedand joined to the intermediate part by means of explosion welding.

The invention accomplishes a number of advantages which are disclosedin, for example, the following description of drawings. A factor havingan especial effect on the good quality of the final product is that theused joining method does not affect the microstructure, integrity orgeometry of the actual electrode surface, because the intermediate partallows the surface of the joint to the substrate material to be farenough from the surface part.

In the following the invention is described by way of example and withreference to the appended drawings, of which

FIG. 1 is a schematical representation of a spark plug according to theinvention,

FIG. 2 is a spark plug of FIG. 1 seen from above, and

FIG. 3 shows the principle of the method according to the invention.

The spark plug 1 shown in FIGS. 1 and 2 comprises three electrodes 2.1,2.2 of which two are so-called ground electrodes 2.2 and one a so-calledcenter electrode 2.1. The operation of the spark plug is known as suchand is not described here as not belonging to the invention. In thespark plug the center electrode 2.1 is supported to the body 3 of thespark plug by means of a separate insulation 4. According to theinvention the spark plug consists of a first part 5 made of a substratematerial, the first part possibly being a part of the body 3 of thespark plug. Further, the electrode consists of a surface part 7, moredurable than the substrate material, the surface part being joined tothe first part of the spark plug via an intermediate part 6 which isalso a part of an electrode 2.1, 2.2. In the invention, the surface part7, which preferably is a metal or metal alloy of the Pt group, is joinedas a thin film first to the more conventional and easier material, suchas a heat-resistant steel or a super alloy, such as an Ni-based superalloy. With the reference to schematic FIG. 3, according to theinvention the joining technology is such that the joint surface 10 isessentially homogenous. Preferably the joining method is explosionwelding. The spark plug body as such, is manufactured in conventionalmanner, made of substrate material. According to the invention a planarblank 8 is formed by joining the surface part 7 to the intermediate part6 making use of explosion welding. The blank may be e.g. a considerablybig plate, from which a part 9 having a suitable form is separated. Thepart 9 consists of the surface part 7 and intermediate part 6. The part9 is fastened to the spark plug body i.e. the first part 5 so that thejoint is made between the first part 5 and the intermediate part 6.

Explosion welding produces a lot of advantages for the production of theelectrode and for properties during use.

-   -   There is no melt zone within the joint, whereby the depletion of        the expensive metal of the Pt group into the substrate can be        minimized, whereby also the said metal film can be thin.    -   Because of the above considerable cost savings can be achieved.    -   There are no brittle phases formed in the transformation zone        near the interface surface between the Pt group metal film and        the substrate material    -   The possibility of getting a very strong, slightly wavy joint        surface.    -   Solid, homogenous and smooth joint surface, no separate beads.    -   The use of melt welding, causing the above-mentioned problems,        can be avoided.

It is possible to use explosion welding for producing a two- ormulti-layer plate, i.e. blank 8, from which parts 9 of suitable size orready electrodes are separated, for example by means of laser cutting,water jet cutting or spark machining, to be further attached to thespark plug 1. These are attached to form, for example, a groundelectrode to the first part 5 of the electrode 2.1, 2.2 in connectionwith the body 3 of the spark plug.

The attaching can be accomplished by using conventional means, such aslaser welding, electron welding or friction welding. Soldering can alsobe used as an attachment means.

Typically, hundreds of electrodes can be produced from an explosionwelded blank 8. Usually, the explosion welded surface does not needpost-machining or grinding. Further, any heat treatment of the electrodeblanks can be easily and inexpensively accomplished as the pieces areseparate and compact in size. The gap is adjusted as necessary when theelectrodes are fastened.

The invention is not limited to the embodiments described here, but anumber of modifications thereof can be conceived of within the scope ofthe appended claims.

1. A method for producing a spark plug having at least two electrodes,in which the electrodes each include at least a base part, made of asubstrate material of the spark plug, and a surface part, made of amaterial more durable than the substrate material, comprising thefollowing steps: forming a blank comprising a surface part and anintermediate part, by joining the surface part to the intermediate partby means of explosion welding, separating a part with a suitable formfrom the blank to form the electrode of the spark plug, wherein the partthat is separated from the blank includes both a portion of the surfacepart and a portion of the intermediate part, and fastening the partseparated from the blank to the base part of the spark plug so that ajoint is made between said base part and said portion of theintermediate part.
 2. A method according to claim 1, wherein theintermediate part of the blank is planar and the surface part of theblank is planar and consists of at least one metal of the Pt group or analloy thereof.
 3. A method according to claim 2, wherein the surfacepart of the blank is formed of powder consisting of at least one metalof the Pt group or an alloy thereof, the powder being simultaneouslysolidified and joined to the intermediate part by means of explosionwelding.
 4. A method for producing a spark plug having a centerelectrode and a ground electrode in which the electrodes each include atleast a base part, made of a substrate material of the spark plug, and asurface part, made of a material more durable than the substratematerial, comprising the following steps: forming a composite blankcomprising a surface part and an intermediate part, by joining thesurface part to the intermediate part by means of explosion welding,separating a first composite member with a suitable form from the blankto form the ground electrode of the spark plug, wherein the part that isseparated from the blank includes both a portion of the surface part anda portion of the intermediate part, fastening the first composite memberto the base part of the ground electrode so that a joint is made betweensaid base part of the ground electrode and said portion of theintermediate part of the first composite member, providing a secondcomposite member that comprises a second surface part made of a materialmore durable than the substrate material of the base part of the centerelectrode and also comprises an intermediate part to which the secondsurface part is joined by explosion welding, and fastening the secondcomposite member to the base part of the center electrode so that ajoint is made between said base part of the center electrode and theintermediate part of the second composite member and the surface part ofthe first composite member is in spaced confronting relationship withthe surface part of the second composite member.